The proposed studies will test several hypotheses regarding melatonin receptors, with special focus on identifying the receptor which mediates circadian responses to the hormone. The pattern of receptor binding will be examined in the brain and other tissues of mice in which the Mel-1a melatonin receptor gene has been disrupted by homologous recombination in embryonic stem cells. Mel-1a "knockout" mice will then be used to determine whether this receptor mediates circadian, vascular, retinal, and longevity responses to melatonin. The Mel-1a gene will be disrupted with a lacZ reporter gene to characterize cellular and developmental patterns of Mel-1a gene expression and to establish if the Mel-1a locus can be used to deliver exogenous genes to the SCN. The melatonin receptor expressed in the human SCN will be identified using molecular, developmental, and pharmacologic approaches.